To me this stuff is fairly simple - but I don't use Facebook anymore fwiw.
There were a bunch of prior attempts at large scale social networking like this and they pretty much all ran into issues with scaling. Facebook handled this adeptly, plus they didn't allow their user-base to turn into a total cesspool through ridiculous customizations + features that made it act like a hookup service ala MySpace, which is why they ate their lunch.
Now the network effects are extreme with nearly a billion served - so it's not exactly hard to keep users' attention when most of their friends and family use the service as well. And I haven't seen any viable competitors come across except Google+ which doesn't seem to be a real threat. So I'm not sure who exactly is nipping at their heels unless we're thinking of a different way of attacking social ala Pinterest (and formerly Instagram :)... which I think is what it will take. You won't outdo them by being a better them. It seems to me most of their innovation has been in a way to set up better ways to monetize or lock in their mass of user data.
It seems crazy that just for 2 examples of youngster having been promoted on the front page of magazines suddenly everyone seem to think only young people can do that.
As far as I can tell, and in fact I don;t care that much, Steve Jobs was not very young, neither is Jeff Bezos. The founder on the web company where I work is look young, but not that young, I'd guess over his forties.
Zack bypassed requirement #1 simply because it received enough funding to not worry about the first problem you have when starting a business (having revenue). He's also smart and probably picked up the required experience on his way.
On the other hand there are plenty of youngsters that fail because of a lack of experience, once confronted with the harsh realities of running a business, and there are also plenty of seniors that fail simply because they are not hungry enough, which limits their creativity.
He can buy/hire people who are good at that.